That manual was written at the height of the oil embargo and the thought was clearly referring to fuel usage and the point of diminishing return on hp versus fuel usage. It was also written by a sailor who had low expectations compared to today
The world was different then, no four-strokes, three brands of engines and I think Chrysler was the only manufacturer of an HT model. Scary thought....Chrysler on the cutting edge of technology. Anyway, the general rule still applies, 1 hp per 500 lbs. The exception being the use of an HT model.